While the endomembrane system, which consists of nuclear envelope, endoplasmic reticulum, Golgi apparatus, secretory vesicles, and plasma membrane, functions in synthesis and exocytosis of secretory products from eukaryotic cells, much remains to be learned about the integrated functions of this system. In the differentiated mammary epithelial cell, exocytotic discharge of the constituents of milk involves net discharge of cell surface membrane. In this proposal continuation studies on the role of endomembranes in synthesis, postsynthetic modification and secretion of the constituents of milk are outlined. Specifically, the proposed study is designed to (1) elucidate the ionic composition of the content of secretory vesicles; (2) compare the rates of appearance and turnover of structural protein and lipid constituents and secretory constituents in components of the endomembrane system; and (3) study the in vitro translation and co- or post-translational modification of the apical plasma membrane-milk lipid globule membrane major coat associated polypeptides. These coat polypeptides are believed to function in membrane recognition and membrane interaction. Approaches followed will emphasize biochemical studies with purified and cytologically defined endomembrane components.